Steam turbines substantially comprise a rotor, which is mounted rotatably about an axis of rotation and comprises rotor blades, and also a housing formed with guide vanes, wherein a flow duct is formed between the rotor and the housing and comprises the guide vanes and rotor blades. Thermal energy of the steam is converted into mechanical energy of the rotor. Various partial turbines are known, these being divided, for example, into high-pressure, intermediate-pres sure and/or low-pressure partial turbines. The division of the partial turbines into a high-pressure, intermediate-pressure and low-pressure part is not uniformly defined among experts. The division depends in any case necessarily on the pressure and the temperature of the inflowing and outflowing steam.
Furthermore, embodiments in which a high-pressure part and an intermediate-pressure part are arranged in a common outer housing are known. Embodiments of this type require two inflow regions arranged tightly next to one another. In this case, it is necessary for rotor-dynamic aspects for the high-pressure inflow and intermediate-pressure inflow to lie tightly against one another, since the axial space is limited. Furthermore, it is more cost-effective if the high-pres sure inflow region and the intermediate-pressure inflow region are arranged tightly next to one another.
Furthermore, it is known to feed the steam to the flow duct via valves. In this case, steam flows through a fast-acting shut-off valve and a control valve and subsequently into an inflow region, and from there into an annular duct. The annular duct has a substantially rotationally symmetrical form about the axis of rotation. The velocities of the steam in the annular duct should be as uniform and low as possible. In the case of two-valve arrangements, i.e. steam flows via two valves and therefore via two inflow regions into the inflow duct, the flow conditions in the annular duct are different to those in the case of one-valve arrangements. In the case of one-valve arrangements, the steam flows via merely one inflow region into the annular duct. The cross section of the annular duct in the case of one-valve arrangements is generally greater than the cross section of the annular duct in the case of a two-valve arrangement. This is effected substantially so that the flow velocities are kept at a low level.
It would be possible to increase the size of the annular duct in the radial direction, but this increases stresses driven by internal pressure in the inner housing. On the other hand, an increase in the wall thickness would lead to stress reduction, which in turn would lead to an increase in the temperature-driven stresses. These two design concepts need to be optimized.